Gross v Net Occupant Load Factors

Most people can recall seeing a sign in a building they’ve visited before that listed the maximum occupancy of a building. We’ve all seen these signs, but who determines the maximum occupancy of a building, and what are some of the rules to follow in making such a determination?

IBC Section 1004 Occupant Load

Usually, it will be the design architect who is responsible for establishing the maximum occupancy of the building they are designing. And the rules for determining an occupant load for a building will be found in Chapter 10 of the International Building Code (IBC), specifically, Section 1004, “Occupant Load.”

In section 1004, various occupant load factors are provided, which are used to calculate how many persons can safely occupy any given building or space within the building. Table 1004.5 introduces the requirement to calculate the occupants based on a “gross” or “net” area. And you wouldn’t know this just by looking at Section 1004 itself, but these terms (“Floor Area, Gross” and “Floor Area, Net”) are defined in Chapter 2 of the code. Floor Area, Gross is partially defined as including accessory areas such as corridors, stairways, ramps, closets, etc., while Floor Area, Net, specifically excludes such areas from the calculation. Why is this?

Gross Floor Area: Primary and Accessory Areas

In reviewing Table 1004.5, it becomes clear that most occupant groups where a gross floor area is required are buildings and spaces where it is more common for such accessory areas to be occupied at the same time as the primary use area. For example, in a shopping mall, where the primary uses are the stores within the mall, it is not unlikely at all that the bathrooms, corridors, and storage areas will all be occupied at once, hence the requirement for a gross area calculation, to make sure that the building’s safety systems account for occupants in all of these accessory areas in addition to the primary use areas. In contrast, for buildings like schools, where it is less likely that the primary and accessory spaces will be occupied at once (for example, a student will either be in their classroom, or in the corridor, or in the bathroom, but will not occupy all at once), a net floor area is applicable, since the accessory areas will not likely be fully occupied at the same time as the primary areas.

In Conclusion

Once you understand the rationale behind assigning floor areas a “gross” or “net” description, it makes sense why the building code requires this determination to be made. Ultimately, any given building or space within a building needs to have the proper safety facilities for the expected number of occupants, and classifying the areas into “gross” or “net” areas will help ensure the building is properly designed for all expected occupants.